The safe disposal of waste materials, particularly hazardous waste materials, is an area of increasing social concern. The manifold benefits of advances in chemistry, medicine, and nuclear technology have been accompanied by the accumulation of large quantities of toxic, infectious and/or radioactive waste materials as well as otherwise inert materials which have been contaminated by such undesirable agents. In some areas, subterranean contamination of the soil by such agents threatens the safety of underground water supplies and other natural resources.
DC arc furnaces have been employed to melt ferrous metals. The use of such furnaces to process waste materials offers several advantages in the disposal of such materials relative to other available disposal technologies such as incineration. These advantages include the ability to separate and recover useful materials from the waste, reduced gaseous emissions, and a more complete breakdown of the waste material into non-hazardous components or into pure elements. Such advantages are described in U.S. Pat. No. 4,431,612 issued Feb. 14, 1984 and in U.S. Pat. No. 3,812,620 issued May 28, 1974, both of which are incorporated by reference herein.
Several of the particular advantages described in the aforementioned patents result from the effects of a DC plasma or arc that is rooted at one end to an electrode and at the other end to a conductive pool of molten material within a conductive hearth. Such an electric arc is referred to as a “transferred” arc, in that, electrical current is transferred between the electrode assembly and the waste material. Another known type of DC arc processing utilizes a “non-transferred” arc wherein an arc is established between two electrodes in the vicinity of the material to be treated so that thermal energy is transferred from the arc to the material primarily by radiation. A transferred arc provides more effective transfer of thermal energy to the waste material and enhanced chemical breakdown of the waste material relative to a non-transferred arc.
In order to establish a transferred arc in accordance with the known methods, it is necessary either (i) to establish a conductive molten pool or “heel” within the hearth prior to adding waste materials to the hearth or (ii) to limit the use of such apparatus to the treatment of waste materials having a sufficient bulk conductivity to establish arcing between the electrode and the conductive material in the pool. The requirement of initially forming a conductive layer in the pool prior to adding heterogeneous waste material can cause difficulties in practice, particularly if the furnace is operated on a batch basis. The alternative requirement of restricting the use of a DC arc to the treatment of electrically conductive waste materials severely limits the usefulness of DC arc processing of waste since many waste materials are not sufficiently conductive to establish a transferred DC arc.
It would be desirable to provide a waste treatment system capable of treating a wide range of waste materials using a transferred DC arc regardless of the electric conductivity of such materials. It would also be desirable to provide such a system in which it is not necessary to form a conductive pool within a hearth prior to the addition of such waste materials. In the processing of material on a batch basis, it would be desirable to provide a system in which it is not necessary to remove any non-conductive surface layer(s) after processing each batch.
A more fundamental limitation of prior systems for destroying waste material in an arc furnace is that such materials must be transported from their respective sites of origin to the furnace in order to be processed. In order to remediate contamination at certain remote sites, it would be prohibitively expensive, hazardous, and/or otherwise impractical to remove the waste material from the contaminated site for transportation to a processing facility such as a furnace. Such contaminated sites include nuclear waste reprocessing and storage facilities, chemical weapons repositories, and other locations wherein undesirable or hazardous materials have been stored or otherwise located underground.
Many such buried waste facilities have proven inadequate to confine their respective waste materials to the originally-intended sites as a result of unstable geology or inadequate containment means. It would be desirable to provide a system that is capable of treating such contaminated sites in order to alleviate the contamination therein without having to remove and to transport large volumes of contaminated soil. It would also be desirable to provide a system for forming stable subterranean barriers to contain such underground contamination and to prevent migration of contaminants beyond the intended site or beyond the current boundaries of such underground contamination at sites that have already been compromised.